1. Field of the Invention
The present invention relates to optical signal processing for the purpose of performing signal pathway switching and light output power adjustment for each wavelength in a WDM (Wavelength Division Multiplexing) system. More particularly, it is related to an optical functional device that uses flat light guides to integrate and miniaturize components that are necessary for the adding and dropping of signals with specific wavelengths, the adjustment and monitoring of light output power for each wavelength, and wavelength dispersion compensation for each wavelength.
2. Description of the Related Art
In recent years, the introduction of WDM systems has been aggressively advanced in order to accommodate for the increase in data traffic. These systems are basically point to point systems. However, with large-scale photonic networks in which WDM systems are connected in a mesh, in order to reduce operation costs through efficient operation of the WDM systems, it will be indispensable in the future to have an optical functional device such as a wavelength selective switch operating as an OADM (Optical Add/Drop Multiplexing) device. An OADM is used for adding and dropping of light signals with specific wavelengths. It will also be necessary for such an optical functional device to provide for the adjustment and monitoring of light output power for each wavelength, and for wavelength dispersion compensation for each wavelength.
FIG. 1 is an example of a case in which a wavelength selective switch is used in a WDM system.
In FIG. 1, wavelength division multiplexed light is propagated in the direction of station M, station N, and station O, which are given reference numerals 1000, 1002 and 1004, respectively. An OADM node 1006 equipped with a wavelength selective switch 1008 is arranged on station N.
In the example of the system shown in FIG. 1, light signals λ1(a)-λ5(a) corresponding to each wavelength λ1-λ5 are contained in the wavelength division multiplexed light from station m.
At station N, the adding and dropping of light signals having required wavelengths from among the above optical signals is performed.
The example in the figure shows a situation in which light signals λ2(a) and λ4(a) with wavelengths λ2 and λ4, respectively, are output to the Drop port, and light signals λ2(b) and λ4(b) having the same wavelengths λ2 and λ4, respectively, are added to the Out port in the direction of the next station, station O.
More specifically, the wavelength division multiplexed light from station M is inputted into the IN port of the wavelength selective switch of station N. The wavelength selective switch outputs the required light signals λ2(a) and λ4(a) to the Drop port. Meanwhile, added signals λ2(b) and λ4(b) are inputted from the Add port into the wavelength selective switch, and outputted to the OUT port in the direction of station O. Therefore, the wavelength division multiplexed light with light signals λ1 (a), λ2(b), λ3(a), λ4(b) and λ5(a) are output to station O.
In this way, the wavelength selective switch in this example drops light signals of required wavelengths of the inputted wavelength division multiplexed light, and adds light signals that are different from the dropped light signals but are at the same wavelengths.
FIG. 2 is a first conventional example of a wavelength selective switch which includes a Micro Electro-Mechanical System (MEMS) 1010 having mirrors 1012 and 1014. The wavelength selective switch also includes a lens 1016 and a diffraction grating 1018. Generally, the MEMS is a mechanical optical switch that electrically controls the angles of the mirrors.
In FIG. 2, the wavelength selective switch is a configuration in which wavelength-multiplexed collimated light that enters from the IN port and the ADD port is branched into each wavelength with the diffraction grating. The MEMS mirrors are in the positions of convergence of all of the wavelengths.
Depending on the angles of the corresponding MEMS mirrors, the light of each wavelength entering from the IN port either heads for the OUT port or is outputted from the DROP port.
The light of the same wavelength as the light outputted from the DROP port, which entered from the ADD port, is wavelength-multiplexed with light that enters from the IN port and heads for the OUT port, and it is outputted from the OUT port.
FIG. 3 is a second conventional example of a wavelength selective switch configuration using arrayed waveguide gratings (AWG) 1020 and 1022, diffraction gratings 1024 and 1026, and MEMS mirrors 1028. FIG. 3 also shows optical circulators 1030 and 1031, and lenses 1032, 1033, 1034, 1036, 1037 and 1038.
The wavelength multiplexed light that entered from the INPUT port and the ADD port is branched by the first AWG 1020 and the second AWG 1022, respectively, into wavelength groups containing multiple wavelengths. They are further branched into each wavelength within each wavelength group by the first diffraction grating 1024 and second diffraction grating 1026, respectively, and they are directed to the MEMS mirrors corresponding to each wavelength.
The MEMS mirrors are configured such that the decision to return signals of light from the first AWG to the first AWG (state 1) or send it to the second AWG (state 2) can be switched by changing their tilting angles.
With the light path when the MEMS mirrors are in state 1, the signals of light with appropriate wavelengths that entered from the IN port are reflected by the MEMS mirrors, and they are returned to the first AWG by way of the first diffraction grating. Therefore, they are included in the wavelength division multiplexed light that passes through the optical circulator 1030 and is outputted from the PASS port (equivalent to the aforementioned OUT port).
On the other hand, with the light path when the MEMS mirrors are in state 2, the first AWG and the second AWG are in the optically connected state, and the signals of light with appropriate wavelengths that entered from the IN port are included in the wavelength division multiplexed light that passes through the second AWG and the optical circulator 1031 by way of the second diffraction grating and are outputted from the DROP port. Moreover, the signals of light with appropriate wavelengths that were sent from the ADD port are included in the wavelength division multiplexed light that is outputted from the PASS port by way of the first diffraction grating, the first AWG, and the optical circulator 1030.
In this way, the device enables the adding and dropping of light of specific wavelengths through a wavelength selective switch.
Here, the wavelength selective switch is comprised of a wavelength branching filter that resolves wavelength division multiplexed light into each wavelength, a light switch that switches the routes of the branched light, and a wavelength combining filter that combines into one the light of each wavelength after the routes are switched.
Furthermore, filters with the same compositions are generally used for the wavelength branching filter and the wavelength combining filter, so this wavelength branching filter and wavelength combining filter are hereafter called the wavelength combining/branching filters.
FIG. 4 is a third conventional example of a wavelength selective switch.
Here, the device illustrated in FIG. 4 is a wavelength selective switch comprised of light guides, and in the explanations below, devices of this type are called waveguide type wavelength selective switches. In contrast to this, wavelength selective switches comprised of diffraction gratings, lenses, MEMS mirrors, etc., as shown in FIG. 2, are called spatial-join type wavelength selective switches.
The waveguide type wavelength selective switch of FIG. 4 uses AWGs for wavelength branching filter 1a and wavelength multiplexing filter 1b, and it uses a Mach-Zehnder interferometer type waveguide switch utilizing thermo-optical effects as light switch 2 (this is called a thermo-optical effect type waveguide switch hereafter). FIG. 4 shows a slab substrate 100, a core 202 and a clad 201 of the device.
Here, spatial-join type wavelength selective switches such as that shown in FIG. 2, have characteristics such that, for example, they use free-space diffraction gratings as wavelength combining/branching filters, they use mechanical switches (such as MEMS) for light route switching, and they use free-space optics systems for optical coupling between optical functional components.
On the other hand, waveguide type wavelength selective switches such as that shown in FIG. 4, have characteristics such that, for example, they monolithically integrate component parts comprised of flat light guides, they use AWGs for wavelength combining/branching filters, they use thermo-photometric effect type waveguide switches for light route switching, and they use waveguides for optical coupling between optical functional components.
With the first conventional example shown in FIG. 2, it is difficult to achieve high resolution and miniaturization, which are required in WDM systems.
In order to increase the resolution with diffraction gratings, it is necessary to increase the diameter of the beams that enter the diffraction gratings, and the device consequently grows in size. The resolution of the diffraction gratings is represented by Nm (N: number of gratings in the beam irradiation region; m: diffraction order). Assuming the angle of incidence to a diffraction grating is vertical and the angle of reflection is θ from the normal line of the primary surface of the diffraction grating, the resolution λ/dλ of the diffraction grating is expressed by the following Formula (1).λ/dλ=Nm=N(a*sin θ/λ)  Formula (1)
Here, a is the interval between grid lines of the diffraction grating, and A is the wavelength of the light.
Here, when θ=15° and λ=1.55 μm, the following Formula (2) is applied.λ/dλ=Na/5.99μm  Formula (2)
Here, if the spacing between reflection mirrors is set to μm=500 μm and the beam diameters at the mirror reflection surfaces are set to Dm=100 μm, in order to accommodate for a WDM system in which the light wavelengths are in the spectrum of 1.55 μm and the wavelength intervals are 0.8 nm with the configuration of the first conventional example, resolution of:λ/dλ=1.55μm/0.8 nm×pm/Dm≈10,000  Formula (3)becomes necessary. Using Formula (2), the beam diameters Dg at the diffraction gratings become large, as expressed by:Dg=Na=λ/dλ×5.99 μm≈6cm.
Therefore, the widths of the combining/branching filters within the device must be at least 6 cm, causing the entire wavelength selective switch to require an even broader width.
As illustrated in FIG. 4, the waveguide type wavelength selective switch is formed on a slab substrate, so it is thin. Because a slab substrate with a thickness of approximately 1 mm is normally used, the chip itself is extremely thin. Therefore, it is possible to narrow the thickness of the entire device after it is housed in a protective case. In contrast to this, as stated previously, it is difficult to make thin the diffraction gratings and lenses used in spatial join type wavelength selective switches, so there is the problem in which the thickness of the entire device becomes large.
Moreover, with the spatial join type wavelength selective switch, it is necessary to precisely center and fixate the lens in five axis directions—the direction of the two axes perpendicular to the optical axis, the direction of the optical axis, and the mutually orthogonal two axes, yaw and pitch or angles. As for optical parts other than the lens, in addition to the above five axes, it is necessary to precisely center and fixate the parts in the six axes of the rotation direction. Therefore, there is the problem that assembly is troublesome in comparison to the waveguide type wavelength selective switch.
This is the same for the lenses and diffraction gratings of the second conventional example shown in FIG. 3.
On the other hand, with the second and third conventional examples using AWGs, the AWG parts used as wavelength combining/branching filters can be designed to be more compact than the case of diffraction gratings in the first conventional example.
Therefore, it is possible to make the wavelength selective switch smaller than the first conventional example, but the insertion loss of AWGs with configurations such as that shown in FIG. 4 is approximately 3 dB each time a light signal passes through. In the second and third conventional examples, light signals of each wavelength pass through AWGs twice—once at the time of branching and once at the time of multiplexing—so the fact that the insertion loss of AWGs becomes as large as approximately 6 dB. This high insertion loss is a problem.
The reason that the insertion loss of AWGs of the configuration used in the second and third conventional examples becomes large will be explained below.
FIG. 5 is a block diagram of the conventional AWG.
In FIG. 5, the conventional AWG is comprised of input waveguide 3, input slab waveguide 4, channel waveguide array 5 comprising multiple channel waveguides, output slab waveguide 6, and multiple output channel waveguides 610.
Input waveguide 3 is for the purpose of guiding the input light from waveguide end face 203 to input slab waveguide 4, and input slab waveguide 4 is for the purpose of distributing the input light to channel waveguide array 5.
Input slab waveguide 4 extends in the direction parallel to the page of FIG. 5, and when light enters input slab waveguide 4 from input waveguide 3, it freely expands and propagates without being confined in the direction parallel to the page of FIG. 5.
In order for light that has freely propagated through input slab waveguide 4 in the direction parallel to the page to reach channel waveguide array 5 and optically couple, the power of the input light is distributed to all of the channel waveguides that constitute channel waveguide array 5.
Channel waveguide array 5 is for the purpose of providing phase shifts to the light that passes through here, and it is formed such that the differences between effective light path lengths of adjacent waveguides are constant.
Therefore, when light propagates through channel waveguide array 5 to the boundary with output slab waveguide 6 from the boundary with input slab waveguide 4, phase shifts corresponding to the wavelengths of the light within each channel waveguide are generated. This phase shift contributes to spectroscopy effects described later.
Output slab waveguide 6 is for the purpose of freely propagating and interfering with light outputted from channel waveguide array 5.
When light of the same phase is outputted from each channel waveguide constituting channel waveguide array 5, light of a given wavelength is focused on the boundary of the output slab and the waveguide positioned vertically in the center in FIG. 5 from among output channel waveguides 610.
This is because the boundary between channel waveguide array 5 and output slab waveguide 6 forms an arc centered on this position in which light is focused, and the light leaving each channel waveguide proceeds directly towards the center of this arc—that is, the center of output channel waveguides 610 positioned in the central region of the vertical direction. The wavelength at this time is called the center wavelength.
In the case in which the wavelength of light is shorter than the center wavelength, the phase of the light outputted from channel waveguide array 5 proceeds to the bottom of the figure. Focusing attention on the position in which the phases of light outputted from each channel waveguide are equal (this is called the equal phase front hereafter), the further they proceed towards the bottom of the figure, the further to the right they are positioned. This is the state in which phases are advancing. Therefore, light that is shorter than the center wavelength is relatively focused at the top.
Conversely, when the wavelength of light is longer than the center wavelength, the phases of the light outputted from the channel waveguides proceed to the top of the figure. Therefore, they are relatively focused at the bottom.
In this way, on lines 611 that connect the boundaries between output slab waveguide 6 and output channel waveguides 610, spectroscopy and conversion are performed as a continuous spectrum in which the top forms short wavelengths and the bottom forms long wavelengths. Moreover, lines 611 that connect the boundaries between output slab waveguide 6 and output channel waveguides 610 form an arc.
Output channel waveguides 610 are for the purpose of cutting out only light of a specific wavelength band from the continuous spectrum focused on arc 611 and guiding it to waveguide end face 204, and they comprise multiple channel waveguides. As previously stated, if the positions on arc 611 are different, then the wavelength band of trimmed light out becomes different.
The spacing between output channel waveguides 610 on arc 611 is proportionate to the wavelengths of outputted light. Therefore, if the output channel waveguides are arranged on arc 611 at equal intervals, then the wavelength intervals of light that is trimmed and outputted also become equally spaced. Moreover, by adjusting the spacing between the output channel waveguides, it is possible to adjust the wavelength spacing of the outputted light.
Furthermore, the configuration of the aforementioned AWGs and the spectroscopy principles thereof are described in, for example, the document, “Meint K. Smit and Cor van Dam, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 2, pp. 236-250 (1996).”
FIG. 6 is a drawing explaining the distribution of light intensity at the output channel waveguide parts.
FIG. 6(a) is an enlarged drawing of part A of FIG. 5, and FIG. 6(b) is an enlarged drawing of part B of FIG. 6(a).
As illustrated in FIG. 6(a), the light that is focused on arc 611 has an intensity distribution that is strong in the central region, as shown by 612, for example, and rapidly weakens towards the edges (vertical in the diagram).
For example, supposing that white light is inputted and the light intensity distribution shown by 612 is the wavelength λc intensity distribution, then light with slightly short wavelength (λc−Δλ) and light with slightly long wavelength (λc+Δλ) result with the same intensity distributions. In the case in which the incident light is white light, the light of this intensity distribution continuously forms lines.
At this time, focusing attention on efficiency of coupling with the output channel waveguides, light with wavelength λc forms a bond with highest efficiency due to the fact that the output channel waveguides and the optical axis are in agreement. In contrast to this, the coupling efficiency of light with a wavelength of λc−Δλ or λc+Δλ is decreased due to the output channel waveguides and the optical axis being misaligned, and the coupling efficiency further diminishes as the wavelength deviates from λc.
Coupling in this case becomes almost the same as Gauss beam coupling.
FIG. 7 shows the loss with respect to wavelength of light that is outputted from the output channel waveguides at this time. This is a graph of the intensity of light outputted from the output channel waveguides with wavelength on the horizontal axis and intensity on the vertical axis (this becomes the same as the spectrum outputted from channel waveguides), and its shape is Gaussian.
However, in a communications system, the transmission property in which the end is roughly flat (called flat top hereafter) is desired. This is because it is desirable for the loss to be roughly equal, even if each wavelength that constitutes wavelength division multiplexed light changes within a given spectrum due to changes in environmental conditions, for example.
Here, conventional technology for the purpose of flat-topping the transmission properties will be explained.
FIG. 8 is a conventional configuration example for the purpose of flat-topping the transmission properties.
In FIG. 8, broad part 301 (multimode waveguide part) is formed on the boundary of input waveguide 3 and input slab waveguide 4, and this performs the flat-topping of the spectrum. The light intensity distribution becomes double peaked (referred to as the “double peak mode” hereafter) at the broad part 301 of the input waveguide.
FIG. 9 is a diagram that describes the light intensity distribution at the output channel waveguide parts corresponding to FIG. 8. FIG. 9(a) is an enlarged drawing of part A of FIG. 8, and FIG. 9(b) is an enlarged drawing of part B of FIG. 9(a).
When light that moves into the input slab waveguide enters the double peak mode, the intensity distribution 612 of light that is focused on output channel waveguides 610 also enters the double peak mode, as illustrated in FIG. 9(a). In other words, the shape of the intensity distribution of light that moves into the input slab waveguide and the shape of the intensity distribution of light that is focused on output channel waveguides 610 are the same.
FIG. 9(b) shows the intensity distributions of light with wavelength Ac, light with slightly short wavelength (λc−Δλ), and light with slightly long wavelength (λc+Δλ). They all form the same shape when the incident light is white light.
The coupling efficiency of this double-peak mode light having wavelengths λc, λc−Δλ, and λc+Δλ with the output channel waveguides becomes constant if the size of the central cavity and the spacing between the two peaks are adjusted.
FIG. 10 is a figure showing the loss with respect to wavelength of light that is outputted from the output channel waveguides when the spacing between the two peaks is adjusted in this way. As illustrated by the curve (b) of FIG. 10, for example, flattop transmission properties are obtained. Moreover, the curve (a) of FIG. 10 has the Gaussian transmission properties shown in FIG. 7.
In this way, by using the structure illustrated in FIG. 8—that is, by forming part 301 that makes the input waveguide into multiple mode—it is possible to achieve flattop type transmission properties.
However, as is clear from the comparison of (a) and (b) in FIG. 10, the loss in the case of (b)—in which part 301 that makes the input waveguide into multiple mode—is greater than that of (a).
This loss is approximately 3 dB in cases in which light passes through AWGs once. In the example of FIG. 3, for example, it passes through the AWGs twice, so this results in a loss increase of approximately 6 dB.